Gordon Brown sent a personal letter to Muammar Gaddafi yesterday calling on Libya to handle "with sensitivity" the return of the Lockerbie bomber Abdelbaset al-Megrahi, who was met with a hero's welcome in Tripoli last night.

A Downing Street source said the letter had been "very short and to the point". It reiterated the British government's insistence that the decision to release Megrahi was made by the Scottish government alone, and called on the Libyans "to act with sensitivity around Megrahi".

"I don't think there is a menu of actions to choose from," a British official said.

Last night's welcome has had one immediate effect on British-Libyan relations: it is now very unlikely that the Duke of York will attend ceremonies next month marking the 40th anniversary of the Libyan revolution.

The Libyans have not yet sent formal invitations and were presumed to be awaiting the outcome of the Megrahi case, but the provisional plan for attendance by Prince Andrew, who has made two official visits in the past two years as well as private trips, has been scrapped. If an invitation is received, the British representation will be less senior.

A flag-waving crowd gathered to cheer Megrahi as he emerged from the plane sent by the Libyan government to collect him from Scotland. However, British officials maintained that it was too early to judge the Libyan response, and they will wait to see whether Gaddafi meets Megrahi in person and whether the released convict, who is believed to be in the terminal stages of prostate cancer, maintains a high profile in the next few days.

"How they act now is going to be viewed very carefully," the Downing Street official said, admitting it had been "a deeply distressing start". In an additional snub to Brown, the Libyan government leaked the letter to local journalists this morning.

Despite the scenes at Tripoli airport last night, it seemed today that Libya was trying to play down Megrahi's return, keeping him out of the public eye and making little official mention of him. It was unclear where he has been taken, and officials made no comment on his whereabouts.

Despite warnings from David Miliband, the foreign secretary, and Downing Street that Libya's behaviour would determine the country's international standing, British officials said there was no talk yet of re-imposing any of the sanctions used against Libya before Gaddafi handed over Megrahi and abandoned a nuclear programme.

David Cameron wrote to the prime minister today to challenge him to reveal his views on the decision to release Megrahi.

The Conservative leader told Brown: "You have not commented on the decision since it was announced yesterday. This morning your foreign secretary refused several requests to say what he thought of the Scottish justice secretary's decision.

"The fact that the decision to release was taken by the Scottish justice secretary does not preclude you, as the prime minister of the United Kingdom, from now expressing your opinion on a subject that is of great public concern, and which affects Britain's international reputation and our relations with our allies.

"It is curious that while others have commented, Britain's own prime minister has not. I hope you will now take the opportunity to do so.

"We are entitled to know what you and your ministers have said to the Libyan authorities on this matter, and to the Scottish justice secretary."

Such claims were "a slur both on myself and the government", Miliband told Radio 4's Today programme. "We have been scrupulous in saying this decision should be made by the Scottish authorities; we have been scrupulous in saying that to the Libyans, we have been scrupulous in saying that to the Americans.

"We certainly welcome the fact that over the past 10 years there have been significant changes in Libya's engagement with the international community. But it is wrong to say that in this case, the British government has somehow put pressure on the Scottish authorities or anyone else."

Miliband refused to say whether he agreed with the Scottish government's decision. He condemned the reception Megrahi received in Tripoli.

"Obviously the sight of a mass murderer getting a hero's welcome in Tripoli is deeply upsetting, deeply distressing, above all for the 270 families who grieve every day for the loss of their loved ones 21 years ago and also for anyone who has an ounce of humanity in them. I think that is the overriding emotion that people will be feeling today," he said.

The Scottish Labour party seized on the decision to release Megrahi as proof that the SNP was "unfit" to govern, prompting claims by the Liberal Democrats that Labour was guilty of "facing two ways" because of the government's arm's-length involvement in Megrahi's release.

Scotland's first minister, Alex Salmond, said he shared Miliband's concerns about the "inappropriate" high-profile welcome received by Megrahi.

He told Today: "I don't think the reception for Mr Megrahi was appropriate in Libya; I don't think that was wise and I don't think that was the right thing to do."

Salmond denied that the decision of his justice secretary, Kenny MacAskill, had damaged the reputation of Scotland. "We're not responsible for the actions of others. I don't think we've damaged Scotland's reputation – on the contrary. We have to do what we think is right and proper and that's what the justice secretary did."

Barack Obama last night denounced Megrahi's release as a mistake and revealed that the US had opened talks with Libya urging the regime to keep the terminally ill man under house arrest until his death.

Asked about objections from the US, Salmond said: "Our relationship with America is a strong and enduring one, it doesn't depend on always reaching agreement. That can't be the case otherwise there would be no point in having our own independent decision-making, our own jurisdiction."